FIG. 1 is a side view showing a structural example of a propeller-type wind turbine for use in a wind turbine generator. A wind turbine 11 is provided with a support 12 fixed to the ground, and a nacelle 13 supported on an upper portion of the support 12. A rotor head 14 is attached to an end of the nacelle 13. A plurality of wind turbine blades 15 is attached to the rotor head 14. Although three wind turbine blades 15 are typically included and thus a configuration including three wind turbine blades will be described herein, the description herein can be applied to cases of including the different number of wind turbine blades 15. A control part 16 controls a blade pitch angle of the wind turbine blade 15 so as to attain a certain rotational speed and output depending on wind conditions. In addition, when an alert is detected, a shutdown control is carried out so as to quickly close the blade pitch angle of the wind turbine blade 15 to immediately stop the wind turbine 11.
When deviating from a normal condition due to external factors, the wind turbine is required to stop until the external factors are eliminated. For example, when a wind speed exceeds a certain condition, when variations in temperature, humidity and system frequency exceed their respective predetermined standards, when an abnormality occurs in a voltage, or when a vibration is generated due to an earthquake, etc., the wind turbine stops for safety in response to a sensor that detects the occurrences and transmits an alert.
The wind turbine stops as follows. The wind turbine converts a force of wind into an electric power during a normal drive thereof. The generated power is converted into an electric power whose characteristics such as the frequency is the same as those of the system power. Thereafter, the generated power is collected for the entire wind farm to which all those wind turbines belong, and then the collected power is interconnected with a system power source.
If any alert is transmitted during the drive, a shutdown sequence, which is configured by a series of controls for completely stopping a wind turbine generation function of the wind turbine, starts in response to a signal indicating the alert. In the shutdown sequence, a break works to exert a breaking force against the rotation of the wind turbine, and then a feathering motion of the pitch starts to move the pitch from a fine side to a feathering position. As a result of the operation, the rotational speed of the wind turbine gradually decreases. When the rotational speed of the wind turbine falls below a predetermined reference value, the wind turbine is blocked out of the power system of the wind farm and thus disconnected. When the wind turbine is disconnected and the pitch reaches the feathering position, the shutdown completes.
The patent document 1 discloses a method for controlling a wind turbine generation system in the case where a system voltage decreases or an overcurrent occurs in a power converter on the system side.